NEW YORK (Reuters) - U.S. stocks edged lower on Monday as caution over the potential for volatility driven by worries about the U.S. "fiscal cliff" dampened enthusiasm at the start of a seasonally strong period for equities.

Investors are betting Congress will reach a deal to avert most of the austerity measures due to come into force at the start of next year. That has led to the best year for stocks since the post-financial crisis rebound. But those gains may be quickly reversed if a deal is not reached soon.

The S&P 500 index posted its biggest drop in more than a month on Friday as a Republican plan to avoid the cliff - $600 billion in tax hikes and spending cuts that could tip the U.S. economy into recession - failed to gain traction on Thursday night.

Sharp moves like that highlight how headlines from Washington can whipsaw markets, especially during the thinly traded period over the Christmas holiday.

Still, with the S&P 500 up 0.7 percent in December and on course for its strongest month since September, some analysts are predicting that stocks will find their footing during a market seasonality known as the "Santa Claus rally."

"Right now we've seen some very constructive action in the market so I think that bodes well for this being a positive seasonal 'Santa' period over the coming seven days," said Ari Wald, a technical analyst at The PrinceRidge Group.

He noted an all-time high in the NYSE advance-decline line, which compares advancing and declining stocks, as indication of strong participation in the rally off November lows.

"Pull-backs are buying opportunities," said Wald. "There has been really great participation on this move, a lot of small- and mid-cap stocks behaving well, pushing out to the upside; we're seeing some good leadership from offensive sectors of the market as well."

A high ratio of advancing stocks to declining issues shows there is broad participation across the equity market.

The Santa seasonality covers the last five trading days of the year and the first two of the new year. Since 1928, the S&P 500 has averaged a gain of 1.8 percent during this period and risen 79 percent of the time, according to data from PrinceRidge.

The S&P 500 is up more than 13 percent for the year, having recovered nearly all the losses suffered in the wake of the U.S. election. The yearly gain would be the best since 2009.

Some U.S. lawmakers expressed concern on Sunday the country would go over the cliff, as some Republicans charged that was President Barack Obama's goal. Talks are stalled with Obama and House of Representatives Speaker John Boehner out of Washington for the holidays.

"It does seem like we are continuing through the same drift of the same thing we've had the past couple of weeks - 'cliff' talk," said Nick Scheumann, wealth partner at Hefty Wealth Partners in Auburn, Indiana.

"You can't trade on what you don't know and we truly don't know what they are going to do," he said.

Congress is expected to return to Washington next Thursday as President Barack Obama returns from a trip to Hawaii. As the deadline draws closer, a 'stop-gap' deal appears to be the most likely outcome of any talks.

Trading volume was muted, with U.S. equity markets closing at 1 p.m. (1800 GMT) ahead of the Christmas Day holiday on Tuesday.

In addition, a number of European markets operated on a shortened session, with other markets closed.

U.S. retailers may not see a sales surge from this weekend as ho-hum discounts and fears about imminent tax hikes and cuts in government spending give Americans fewer reasons to open their wallets in the last few days before Christmas.

Aegerion Pharmaceuticals Inc said the U.S. Food and Drug Administration approved Juxtapid capsules in patients with homozygous familial hypercholesterolemia, but will conduct a post-approval study to test long-term safety and efficacy. Shares fell 1.8 percent to $25.25.

Herbalife Ltd dipped 4.4 percent to $26.06 after the company said it expects to exceed its previously announced repurchase authorization guidance and has retained Moelis & Company as its strategic adviser. The declines put the stock on track for a ninth straight decline.

Yum Brands Inc advanced 1.8 percent to $65.01 after Shanghai's food safety authority said the level of antibiotics and steroids in the company's KFC chicken was within official limits.